Oxidation of alkylaromatic compounds

ABSTRACT

A process for the oxidation of alkylaromatic compounds is provided. The process comprises oxidizing a substrate alkylaromatic compound with a peroxygen compound in the presence of a source of cerium (III) or (IV), a source of bromide and a carboxylic acid or anhydride solvent. Preferably, the source of cerium is cerium (III) acetate, the source of bromide is sodium bromide and the solvent is acetic or propionic acid. The process is particularly suited to the selective oxidation of methylaromatic compounds to the corresponding aldehydes.

This is the US National Stage Application of PCT/GB94/02080 filed Sep.26, 1994 now WO95/09139 published Apr. 6, 1995.

This invention concerns a process for the oxidation of alkylaromaticcompounds. More specifically, this invention concerns a catalysedprocess for the oxidation of alkylaromatic compounds with peroxygencompounds.

The oxidation of alkylaromatic compounds is a desirable reaction inorganic chemistry because it permits the conversion of a readilyavailable organic substrate having only limited reactivity into acompound including more reactive functional groups and hence havinggreater reactivity. Oxidation of alkylaromatic compounds commonlyinvolves the oxidation of an alkyl hydrogen which requires the use of arelatively strong oxidant. It is therefore readily apparent that such anoxidant can often readily be employed for the oxidation of other moreeasily oxidised functional groups such as alcohols.

Many systems have been proposed for carrying out the oxidation ofalkylaromatic compounds, including the use of transition metal oxidessuch as manganese dioxide, potassium permanganate and chromium oxide,and other oxidants such as organic peracids and air or gaseous oxygen.One particularly desirable oxidant that has been employed comprises anaqueous solution of hydrogen peroxide. In order to successfully employhydrogen peroxide in the oxidation of alkylaromatic compounds it isoften necessary to employ some other component such as a catalyst, forexample, cobalt species, or a co-reagent, such as hydrogen bromide.However, it remains desirable to identify alternative and furtheroxidation systems for the oxidation of alkylaromatic compounds.

The oxidation of an alkylaromatic can be regarded as proceeding via anumber of stages i.e. alkyl→alcohol→carbonyl compound→carboxyl compound(if appropriate). In addition, the presence of additional reagents maylead to the formation of other compounds, for example mono- and/ordi-brominated species in the case of a bromide co-reagent. Thesecompounds can sometimes be easily oxidised and/or hydrolysed to producealcohols, carbonyl or carboxyl compounds. It will be recognised thatonce an alcohol or carbonyl compound, especially an aldehyde has beenformed, the oxidising conditions can often result in this compound beingfurther oxidised. For this reason, it is often difficult to halt theoxidation at one of the intermediate oxidation products, and on accountof this, such intermediate compounds can command a premium price. Itwould therefore be desirable to identify an oxidation system foralkylaromatic compounds that was effective at producing suchintermediate products and/or which produced compounds that could beconveniently converted to such intermediate compounds.

It is an object of the present invention to provide an alternative andfurther system for the oxidation of alkylaromatic compounds.

It is a further objective of at least some embodiments of the presentinvention to provide a process for the oxidation of alkylaromaticcompounds that is effective at producing intermediate oxidation productsand/or which produces compounds that can be conveniently converted tosuch intermediate compounds.

According to the present invention, there is provided a catalysedprocess for the oxidation of alkylaromatic compounds with a peroxygencompound in a reaction medium, characterised in that the catalystcomprises a source of cerium (III) or (IV), the reaction mediumcomprises a source of bromide ions and a solvent selected from the groupconsisting of carboxylic acids and anhydrides, and the alkylaromaticcompound comprises an alkyl hydrogen atom, and optionally a hydroxylgroup, bonded to the carbon alpha to the aromatic ring.

According to a second aspect of the present invention, there is provideda process for the selective oxidation of methylaromatic compounds toaldehydes with a peroxygen compound in a reaction medium, characterisedin that the catalyst comprises a source of cerium (III) or (IV), and thereaction medium comprises a source of bromide ions and a solventselected from the group consisting of carboxylic acids and anhydrides.

Sources of cerium that can be employed in The catalyst system and theprocess according to the present invention include cerium salts andcerium complexes. The sources of cerium are usually introduced into thereaction medium in oxidation states (III) or (IV). However, it will berecognised that a cerium source in a lower oxidation state can beintroduced into the reaction medium, with oxidation in situ occurring toproduce a cerium species in oxidation state (III) or (IV). In manyinstances, however, the cerium source is cerium (III). Examples ofsuitable cerium salts that can be employed include oxides, hydroxides,halides, carbonates, sulphates, acetates and nitrates. The mostpreferred cerium source is cerium (III) acetate.

In the process according to the present invention, the source of ceriumis usually present in a mole ratio of substrate to cerium of less thanabout 5000:1, preferably from about 3000:1 to about 10:1, andparticularly preferably from about 2500:1 to about 1500:1. Although itwill be recognized that the cerium source can be present in an amountgreater than this, it is believed that the use of such a greater amountis not necessary, and therefore represents unnecessary expense.

The source of bromide ions that is employed in the process according tothe present invention can be selected from the group consisting ofelemental bromine, hydrogen bromide and bromide salts. Suitable bromidesalts include alkali metal bromides, particularly sodium and potassiumbromide, alkaline earth metal bromides such as magnesium and calciumbromide and amine-derived salts such as quaternary ammonium bromides andammonium bromide. In some embodiments of the present invention, thesource of bromide ions comprises cerium bromide, and this can thereforeserve, at least partly, as both cerium and bromide ion source.

The mole ratio of cerium source to bromide source is usually selected inthe range from about 10:1 to about 1:3000, preferably from about 1:100to about 1:1500. In certain embodiments of the present invention, goodresults have been achieved employing a mole ratio of cerium source tobromine source in the range of from about 1:500 to 1:1000.

The solvent in the process according to the present invention comprisesa carboxylic acid or carboxylic anhydride. Usually, the acid oranhydride will contain from 1 to about 6 carbon atoms, and suitableexamples include acetic anhydride, acetic acid and propionic acid. Themost preferred solvent is acetic acid. The weight ratio of solvent tosubstrate can be selected from a very wide range depending, for example,on the solubility of the substrate, but will often be in the range fromabout 100:1 to about 1:5.

It will be recognised that it is possible to employ additional solvents.Such additional solvents can be present at a wide weight ratio to thecarboxylic acid or anhydride solvent, depending for example on thesolubility of the substrate. The weight ratio of carboxylic acid oranhydride to additional solvent is often selected in the range of fromabout 1:10 to about 10:1, often from 1:2 to about 8:1, usually fromabout 2:1 to about 6:1. The additional solvents are usually selected tobe resistant to oxidation, and examples include chlorinated solventssuch as dichloromethane, 1,2-dichloroethane, chloroform and carbontetrachloride, oxidation resistant alcohols such as t-butanol, esterssuch as ethyl acetate and isopropyl acetate, nitriles such asacetonitrile, amides such as dimethyl formamide, and ethers such as1,4-dioxane and tetrahydrofuran.

The reaction medium can comprise water prior to the addition ofperoxygen compound, particularly when a carboxylic acid is employed assolvent. When this is the case, the amount of water is often from 5% to35%, preferably from 15% to 25% by weight, based on the weight ofcarboxylic acid employed. In some embodiments of the present invention,the presence of water in the reaction medium prior to the addition ofperoxygen compound has been found to improve the selectivity of theprocess to aldehyde, and reduce the selectivity to monobrominatedproduct.

Peroxygen compounds that can be employed in the process according to thepresent invention comprise hydrogen peroxide, urea peroxide, persaltsincluding sodium percarbonate and sodium perborate, and peracids, forexample Caro's acid or peracetic acid. Preferably, the peroxygencompound is an aqueous solution of hydrogen peroxide.

The alkylaromatic compounds which can be oxidised by the process of thepresent invention are those which comprise at least one alkyl,preferably a (C₁ -C₆) alkyl, substituent having at least one hydrogenatom or hydroxy group at the alpha position relative to the aromaticring. Although higher alkyl substituents may be oxidised by the processof the present invention, such as those having up to 30 carbon atoms,(C₁ -C₆) alkyl substituents are preferred. From the preferred group ofsubstituents, straight chain (C₁ -C₆) alkyl substituents and branchedchain alkyl substituents having less than 5 carbons are most preferred.Examples of alkylaromatic compounds which can be oxidised by the presentinvention include alkylbenzenes, such as toluene, ethylbenzene,p-t-butyltoluene, cumene, o-, m- or p-xylenes, o-, m- orp-diethylbenzenes and polynuclear alkylaromatic compounds such as themono-, di- and tri-alkyl naphthalenes, e.g. methyl naphthalenes, ethylnaphthalenes and dimethylnaphthalenes.

The alkyl substituent or substituents of the alkylaromatic compounds canbe any substituted alkyl which may be oxidised to an alcohol, a ketoneor carboxylic acid as may be appropriate. Such substituted alkylscommonly have at least one hydrogen atom, and optionally a hydroxygroup, at the alpha position relative to the aromatic ring. Alkylssubstituted with at least one phenyl-, hydroxy-, halo- or oxy-substituent are examples of the substituted-alkylaromatic compoundswhich may be oxidised by the process of the present invention. Forexample, when the alkyl- substituent is methyl-, the methyl- may be amono- or di-substituted methyl- of the formula --CHR¹ R² where R¹ and R²are independently selected from the group consisting of H-, substitutedor unsubstituted phenyl-, --OH and --hal (-hal is -F, -Cl, -Br or -I).Examples of substituted-alkylaromatic compounds are diphenylmethane anddiphenylethane. Preferably, the alkyl group is a methyl group.

The alkylaromatic compounds that can be oxidised by the processaccording to the present invention can comprise an aromatic ring whichis substituted with one or more substituents. The range of substituentsthat can be present will depend on the presence or absence of a hydroxygroup at the position alpha to the aromatic ring, and on the position ofsubstitution, i.e. ortho, meta or para to the alkyl group. When ahydroxy group is present alpha to the aromatic ring, the substituent canbe one or more of nitro, alkyl, halo, aldehyde, ketone, carboxylate orsulphonic acid groups, and preferably alkyl, and the substituent can bepresent at any position on the ring.

When a hydroxy group alpha to the aromatic ring is not present, thenature of the substituent will depend on the position of substitution.Substituents meta to the alkyl group can be selected from the above listfor when the hydroxy group is present. Substituents ortho and/or para tothe alkyl group must not be such that the aromatic ring is net stronglydeactivated, such as by the presence by themselves of strongly electronwithdrawing groups such as nitro or aldehyde groups, although it isbelieved that the effect of such groups may be ameliorated by thepresence of one or more activating, electron donating groups such asalkoxy or amino groups. The aromatic ring must not be net stronglyactivated to ring bromination, such as by the presence by themselves ofalkoxy and amino groups, although it is believed that the effect of suchgroups can be ameliorated by the presence of one or more deactivating,electron withdrawing groups such as nitro, aldehyde or carboxyl groups.In many embodiments, substituents ortho and/or para to the alkyl groupwhen a hydroxy is not present alpha to the aromatic ring are selectedfrom activating or neutral groups such as alkyl and halo groups.

The peroxygen compound can be introduced into the reaction medium instoichiometric, sub-stoichiometric or greater than stoichiometricamounts, based on the mole ratio of peroxygen compound to alkylaromatic.It may be preferable to employ a sub-stoichiometric amount of peroxygencompound when the substrate is particularly sensitive to furtheroxidation. In most embodiments, however, it is preferred to employ atleast a stoichiometric amount of peroxygen compound, and often peroxygencompound an excess of the stoichiometric amount, such as up to 10 molesof peroxygen compound per mole of alkylaromatic, i.e. up to an excess of9 times, and preferably from about 1.5 to about 5 moles of peroxygencompound per mole of alkylaromatic, i.e. an excess of about 0.5 to about4 times above the stoichiometric amount.

When the peroxygen compound is hydrogen peroxide, it is preferablyintroduced into the reaction medium in the form of a concentratedaqueous solution, often comprising from about 25 to about 70% w/w, andfrequently from about 30 to 50% w/w hydrogen peroxide.

When the peroxygen compound is a peracid and particularly peraceticacid, it is conveniently introduced either as an equilibrium aqueoussolution or as a distilled product comprising a solution of the peracidin the corresponding acid which is substantially free of water andhydrogen peroxide. The concentration of peracid is often in the rangefrom about 10% to about 50% w/w, preferably 30% to about 45% w/w.

When the peroxygen compound comprises urea peroxide, sodium percarbonateor sodium perborate, it is conveniently introduced as a solid, anaqueous solution or as an aqueous slurry,

Preferably, the peroxygen compound is introduced into the reactionmedium which contains both the substrate and catalyst system, andparticularly preferably it is introduced gradually, for example over aperiod of from 15 minutes to 4 hours. In certain embodiments of thepresent invention, a plurality of peroxygen compound additions, withoptionally a plurality of additions of source of bromide ions, atintervals during the reaction can be employed.

The process according to the present invention is usually carried out atelevated temperature, typically from 50° C. up to the reflux temperatureof the reaction medium, and particularly from about 60° to about 85° C.Particularly for substrates which boil under standard atmosphericpressure at lower temperatures than the desired reaction temperature,the reaction may be conducted at an elevated pressure selected so as topermit the desired temperature to be attained, but of course the higherboiling substrates may likewise be reacted at elevated pressure ifdesired.

In certain embodiments of the present invention, in addition to theoxidised alkylaromatic compounds, the process produces significantquantities of monobrominated alkylaromatic compounds. Although it isdesirable to minimise the quantity of brominated products, theirpresence is not actually too detrimental to the viability of theprocess, especially when they can be converted into a carbonyl compoundor alcohol. Such a conversion may be effected by acid or alkalicatalysed hydrolysis, followed if desired by further oxidation.

The product(s) of the process according to the present invention can beseparated from the reaction medium by conventional means well known tothose skilled in the art depending on the physical form of the productat the temperature the separation is to occur. If the product is asolid, separation will often be achievable by filtration orcentrifugation. If the product is a liquid, separation will often beachievable by distillation, solvent extraction or an alternative methodsuch as column chromatography.

According to a preferred aspect of the present invention, there isprovided a catalysed process for the oxidation of alkylaromaticcompounds with a peroxygen compound in a reaction medium, characterisedin that the catalyst comprises cerium triacetate, the reaction mediumcomprises sodium bromide and acetic acid, and the alkylaromatic compoundcomprises a methyl group.

Having described the invention in general terms, specific embodimentsthereof are described in greater detail by way of example only. Allyields quoted are based on the weight of substrate added.

COMPARISON 1

4-t-butyltoluene (4.4 g, 30 mmol), sodium bromide (0.76 g) and aceticacid (60 g) were charged to a reactor and heated to 70° C. withstirring. To this reaction medium was added 35% aqueous hydrogenperoxide solution (11.65 g, 120 mmol) over a period of 3 hours using aperistaltic pump. The reaction was continued for a period of 2 hoursafter completion of the hydrogen peroxide addition, and then thereaction medium analysed by gas chromatography and HPLC.

The results of the analysis showed that only 29% of the 4-t-butyltoluenewas converted, yielding 5.4% 4-t-butylbenzaldehyde, 6.6%4-t-butylbenzylbromide, 9.2% 4-t-butylbenzylacetate and 3.8%4-t-butylbenzyl alcohol. This represents a selectivity to aldehyde ofonly 19%, and a total selectivity to desired products (aldehyde, alcoholand bromide) of 55%.

COMPARISON 2

4-t-butyltoluene (4 g, 27 mmol) and acetic acid (60 g) were charged to areactor and heated to 70° C. with stirring. To this reaction medium wasadded 35% aqueous hydrogen peroxide solution (5.2 g, 53 mmol) over aperiod of 1.5 hours using a peristaltic pump. The reaction was continuedfor a period of 4.5 hours after completion of the hydrogen peroxideaddition, and then the reaction medium analysed by gas chromatographyand HPLC.

The results of the analysis showed that none of the 4-t-butyltoluene wasconverted, indicating that no reaction had taken place.

COMPARISON 3

4-t-butyltoluene (4 g, 27 mmol), sodium bromide (0.5 g), ceriumtriacetate (0.2 g) and t-butanol (60 g) were charged to a reactor andheated to 70° C. with stirring. To this reaction medium was added 35%aqueous hydrogen peroxide solution (11.65 g, 120 mmol) over a period of3 hours using a peristaltic pump. The reaction was continued for aperiod of 3 hours after completion of the hydrogen peroxide addition,and then the reaction medium analysed by gas chromatography.

The results of the analysis showed that no substrate had been converted,indicating that no reaction had taken place.

EXAMPLE 4

The procedure of Comparison 1 was followed, with the addition of 0.015 gof cerium triacetate to the reaction medium prior to heating.

The results of the analysis showed that 60% of the 4-t-butyltoluene wasconverted, yielding 29% 4-t-butylbenzaldehyde, 7.2%4-t-butylbenzylbromide, 14% 4-t-butylbenzylacetate, 7.1% 4-t-butylbenzylalcohol and 2.3% 4-t-butyl benzoic acid. This represents a selectivityto aldehyde of 48%, and a total selectivity to desired products(aldehyde, alcohol and bromide) of 72%.

EXAMPLE 5

The procedure of Example 4 was followed, except that the acetic acid wasreplaced with a mixture of 48 g acetic acid and 12 g t-butanol.

The results of the analysis showed that 61% of the 4-t-butyltoluene wasconverted, yielding 30% 4-t-butylbenzaldehyde, 7%4-t-butylbenzylbromide, 9% 4-t-butylbenzylacetate, 6% 4-t-butylbenzylalcohol and 4% 4-t-butylbenzoic acid. This represents a selectivity toaldehyde of 50%, and a total selectivity to desired products (aldehyde,alcohol and bromide) of 71%.

EXAMPLE 6

The procedure of Example 4 was followed, except that the acetic acid wasreplaced with 60 g propionic acid.

The results of the analysis showed that 49% of the 4-t-butyltoluene wasconverted, yielding 24% 4-t-butylbenzaldehyde, 10%4-t-butylbenzylbromide, 5.2% 4-t-butylbenzylpropionate, 6%4-t-butylbenzyl alcohol and 5.1% 4-t-butylbenzoic acid. This representsa selectivity to aldehyde of 49%, and a total selectivity to desiredproducts (aldehyde, alcohol and bromide) of 82%.

EXAMPLE 7

p-xylene (2.9 g 27 mmol), sodium bromide (0.5 g), cerium triacetate (0.2g) and acetic acid (50 g) were charged to a reactor and heated to 70° C.To this reaction medium was added 35% aqueous hydrogen peroxide solution(6.3 g, 67 mmol) over a period of 1.5 hours using a peristaltic pump.The reaction was then continued for a period of 1 hour after completionof the hydrogen peroxide addition, when a further 0.5 g of sodiumbromide was added, and a further 6.3 g of 35% aqueous hydrogen peroxidesolution was added over 1.5 hours via a peristaltic pump. After thecompletion of the second hydrogen peroxide addition, the reaction mediumwas analysed by gas chromatography and HPLC.

The results of the analysis showed that 80% of the xylene was converted,yielding 61% 4-methylbenzaldehyde and 6.6% 4-methylbenzylbromide. Thisrepresents a selectivity to aldehyde of 76%, and a selectivity todesired products of 84%.

EXAMPLE 8

4-chlorotoluene (3.4 g 27 mmol), sodium bromide (4.9 g), ceriumtriacetate (0.6 g) and acetic acid (50 g) were charged to a reactor andheated to 70° C. To this reaction medium was added 35% aqueous hydrogenperoxide solution (6.3 g, 67 mmol) over a period of 1.5 hours using aperistaltic pump. The reaction was then continued for a period of 1 hourafter completion of the hydrogen peroxide addition and then the reactionmedium was analysed by gas chromatography and HPLC.

The results of the analysis showed that 26% of the 4-chlorotoluene wasconverted, yielding 52% 4-chlorobenzaldehyde and 48%4-chlorobenzylbromide. This represents a selectivity to aldehyde of 52%,and a selectivity to desired products of 100%.

EXAMPLE 9

4-t-butyltoluene (27 mmol), cerium acetate (0.6 mmol), sodium bromide (5mmol) and acetic acid (50 g) were charged to a reactor and heated to 70°C. with stirring. To this reaction medium was added 35% aqueous hydrogenperoxide solution (54 mmol H₂ O₂) over a period of I hour using aperistaltic pump. The reaction was continued for a period of 1 hourafter completion of the hydrogen peroxide addition, and then thereaction medium sampled and analysed by gas chromatography and HPLC. Afurther 54 mmol H₂ O₂ as 35% w/w aqueous solution was added to thereaction medium over 1 hour, the reaction continued for a further 1hour, when the reaction medium was sampled and analysed. Again, afurther 54 mmol H₂ O₂ as 35% w/w aqueous solution was added to thereaction medium over 1 hour, the reaction continued for a further 1hour, giving a total reaction time of 6 hours from the commencement ofthe first hydrogen peroxide addition, when the reaction medium wassampled and analysed. The results of the analysis are given below.

    ______________________________________                                        Analysis time    2 hours   4 hours 6 hours                                    ______________________________________                                        Substrate conversion (%)                                                                       30        50      55                                         Aldehyde Yield (%)                                                                             17.6      28      36.4                                       Aldehyde Selectivity (%)                                                                       58.7      56      66.2                                       Monobromide yield (%)                                                                           6.3       5       1.8                                       Monobromide selectivity (%)                                                                    21        10       3.3                                       Alcohol yield (%)                                                                               3.8       7       8.1                                       Alcohol selectivity (%)                                                                        12.7      14      14.7                                       Acetate yield (%)                                                                               2.0       9       8.3                                       Acetate selectivity (%)                                                                         6.7      18      15.1                                       Total selectivity to desired                                                                   92.4      80      83.7                                       products (%)                                                                  ______________________________________                                    

EXAMPLE 10

The general method of Example 9 was followed, except that NaBr (2.5mmol) was added to the reaction medium immediately prior to thecommencement of the second and third hydrogen peroxide additions.

The results of the analysis are given below.

    ______________________________________                                        Analysis time    2 hours   4 hours 6 hours                                    ______________________________________                                        Substrate conversion (%)                                                                       30        57      70                                         Aldehyde Yield (%)                                                                             17.6      38      50                                         Aldehyde Selectivity (%)                                                                       58.7      67      71                                         Monobromide yield (%)                                                                          8.4        6      3                                          Monobromide selectivity (%)                                                                    28.0      10      4                                          Alcohol yield (%)                                                                              0.8        7      5                                          Alcohol selectivity (%)                                                                        2.7       12      7                                          Acetate yield (%)                                                                              2.2       --      8                                          Acetate selectivity (%)                                                                        7.3       --      11                                         Total selectivity to desired                                                                   89.4      89      82                                         products (%)                                                                  ______________________________________                                    

EXAMPLES 11, 12 and 13

In Example 11, 4-t-butyltoluene (27 mmol), cerium acetate (0.6 mmol),sodium bromide (5 mmol) and acetic acid (50 g) were charged to a reactorand heated to 70° C. with stirring. To this reaction medium was added35% aqueous hydrogen peroxide solution (54 mmol H₂ O₂) over a period of1 hour using a peristaltic pump. The reaction was continued for a periodof 5 hours after completion of the hydrogen peroxide addition, and thenthe reaction medium sampled and analysed by gas chromatography and HPLC.In Example 12, the method of Example 11 was followed, except that 5 gwater was added to the reaction medium prior to heating to 70° C. InExample 13, the method of Example 12 was followed, except that 10 gwater was added.

The results are given below.

    ______________________________________                                        Example Number:  11        12     13                                          ______________________________________                                        Substrate conversion (%)                                                                       41.4      41.4   41.0                                        Aldehyde Yield (%)                                                                             20.5      26.1   28.0                                        Aldehyde Selectivity (%)                                                                       49.5      63.0   68.0                                        Monobromide yield (%)                                                                          13.9       2.6    0.0                                        Monobromide selectivity (%)                                                                    33.6       6.2    0.0                                        Alcohol yield (%)                                                                               3.8       9.1   10.5                                        Alcohol selectivity (%)                                                                         9.0      21.9   25.6                                        Acetate yield (%)                                                                               3.2       3.6    2.5                                        Acetate selectivity (%)                                                                         7.7       8.6    6.0                                        Total selectivity to desired                                                                   92.1      91.1   93.6                                        products (%)                                                                  ______________________________________                                    

The results of Examples 4 to 13 above clearly show the benefit of theinstant invention in providing a process for the oxidation ofalkylaromatic compounds. The results obtained were significantly betterthan those achieved in comparisons 1 to 3 when at least one of ceriumsource, bromide source or carboxylic acid/anhydride solvent was omitted.The result of Example 8 showed that a deactivated substrate could beoxidised in good selectivity to desired products by the process of theinstant invention. The result of Example 9 demonstrates that the use ofseveral peroxygen additions during the reaction period can increase thesubstrate conversion. The result of Example 10 demonstrates that the useof several peroxygen additions and several source of bromide additionsduring the reaction period can significantly increase substrateconversion, and also increase the selectivity to aldehyde. The resultsof Examples 11, 12 and 13 clearly demonstrate the benefits that can beobtained by adding water to the reaction medium prior to the addition ofthe peroxygen compound, particularly in Example 13 where a highselectivity to aldehyde was achieved, with no monobrominated productbeing detected.

We claim:
 1. A catalysed process for the oxidation of alkylaromaticcompounds with a peroxygen compound in an aqueous reaction medium,wherein the catalyst comprises a source of cerium (III) or (IV), thereaction medium comprises a source of bromide ions and a solventselected from the group consisting of carboxylic acids and anhydrides,and the alkylaromatic compound comprises an alkyl hydrogen atom bondedto the carbon alpha to the aromatic ring, whereby said process iseffective at producing aldehyde or lower oxidized intermediate oxidationproducts in preference to carboxylic acid.
 2. A process according toclaim 1, wherein the alkyl substituent of the alkylaromatic compoundcomprises from 1 to 6 carbon atoms.
 3. A process for the selectiveoxidation of methylaromatic compounds to aldehydes with a peroxygencompound in an aqueous reaction medium, characterised in that thecatalyst comprises a source of cerium (III) or (IV), and the reactionmedium comprises a source of bromide ions and a solvent selected fromthe group consisting of carboxylic acids and anhydrides.
 4. A processaccording to claim 1 or claim 3, wherein the source of cerium is in theoxidation state cerium (III).
 5. A process according to claim 4, whereinthe source of cerium is selected from the group consisting of ceriummetal, oxides, hydroxides, halides, carbonates, sulphates, acetates andnitrates.
 6. A process according to claim 1 or claim 3, wherein thesource of cerium comprises cerium (III) acetate.
 7. A process accordingto claim 1 or claim 3, wherein the source of bromide ions is selectedfrom the group consisting of bromine, hydrogen bromide, sodium bromide,potassium bromide, magnesium bromide, calcium bromide, cerium bromideand ammonium bromide.
 8. A process according to claim 7, wherein thesource of bromide comprises sodium bromide.
 9. A process according toclaim 1 or claim 3, wherein the carboxylic acid or carboxylic anhydridesolvent comprises from 1 to 6 carbon atoms.
 10. A process according toclaim 9, characterised in that the solvent comprises acetic acid orpropionic acid.
 11. A process according to claim 1 or claim 3, whereinthe mole ratio of alkylaromatic:cerium source is less than about 5000:1.12. A process according to claim 11, wherein the mole ratio ofalkylaromatic:cerium source is from about 3000:1 to about 10:1.
 13. Aprocess according to claim 1 or claim 3, wherein the mole ratio ofcerium source to bromide ion source is selected in the range from about10:1 to about 1:3000.
 14. A process according to claim 1 or claim 3,wherein the peroxygen compound is selected from the group consisting ofhydrogen peroxide, urea peroxide, sodium percarbonate, sodium perborateand peracids.
 15. A process according to claim 1 or claim 3, wherein theperoxygen compound comprises hydrogen peroxide.
 16. A catalysed processaccording to claim 1, wherein the catalyst comprises cerium triacetate,the reaction medium comprises sodium bromide and acetic acid, and thealkylaromatic compound comprises a methyl group.
 17. A process accordingto claim 1 or claim 3, wherein the peroxygen compound is introduced intoa reaction medium which contains both the substrate and catalyst system,and wherein said reaction medium comprises a mixture of carboxylic acidand water prior to the commencement of the introduction of the peroxygencompound.
 18. A process according to claim 17, wherein the amount ofwater in the reaction medium prior to the addition of the peroxygencompound is from 5 to 30% by weight, based on the weight of carboxylicacid.
 19. A process according to claim 1 or claim 3, wherein thearomatic ring of the alkyl aromatic or methyl aromatic compound isadditionally substituted by one or more alkyl or halo groups.
 20. Aprocess according to claim 1 or claim 3, wherein a plurality ofadditions of peroxygen compound at intervals during the reaction periodare employed.
 21. A process according to claim 20, wherein a pluralityof additions of source of bromide ions at intervals during the reactionperiod are employed.
 22. A process according to claim 12, wherein themole ratio of alkylaromatic:cerium source is from about 2500:1 to about1500:1.
 23. A process according to claim 13, wherein the mole ratio ofcerium source to bromide ions source is from about 1:100 to about1:1500.
 24. A process according to claim 1 or claim 3 wherein thearomatic ring of the alkyl aromatic or methyl aromatic compound isadditionally substituted by an alkyl group.